Following closer

[henry]

@jjn912

Do you think the heat rejected out of the back of the car has an effect?

The exhaust and cooling vents are outputting around 700 kW into the area bounded by the floor, wing and rear tyres, plus some outboard from the brakes. I did a cursory calc and got an average 10°C rise for a plume of car cross section at 60 m/s Since this will lower the air density I assume it reduces downforce.

[DiogoBrand]

@jjn912

Do you think the heat rejected out of the back of the car has an effect?

The exhaust and cooling vents are outputting around 700 kW into the area bounded by the floor, wing and rear tyres, plus some outboard from the brakes. I did a cursory calc and got an average 10°C rise for a plume of car cross section at 60 m/s Since this will lower the air density I assume it reduces downforce.

Wouldn't 700kW of heat mean 0% engine efficiency?

[rscsr]

@jjn912

Do you think the heat rejected out of the back of the car has an effect?

The exhaust and cooling vents are outputting around 700 kW into the area bounded by the floor, wing and rear tyres, plus some outboard from the brakes. I did a cursory calc and got an average 10°C rise for a plume of car cross section at 60 m/s Since this will lower the air density I assume it reduces downforce.

Wouldn't 700kW of heat mean 0% engine efficiency?

no, about 33%.

[Tommy Cookers]

about 50%

[henry]

@jjn912

Do you think the heat rejected out of the back of the car has an effect?

The exhaust and cooling vents are outputting around 700 kW into the area bounded by the floor, wing and rear tyres, plus some outboard from the brakes. I did a cursory calc and got an average 10°C rise for a plume of car cross section at 60 m/s Since this will lower the air density I assume it reduces downforce.

Wouldn't 700kW of heat mean 0% engine efficiency?

No.

The fuel flow rate gives around 1250kW (Andy Cowell's number)

If we assume 50% efficiency that’s 625 not leaving the crank but exiting out of the exhaust and radiators. Of the 625 that does some is lost in the transmission, some in the CU, some in the ES etc.

I used 700 because it’s a nice round number. It might be 600 or 800 doesn’t make a huge difference.

[jjn9128]

1250kW seems high... that's ~1680bhp. That said a 10°C average increase may be about right.

I'd say the greatest heat source is the exhaust, which is blowing at 400-600°C, but the exhaust plume is fairly quickly swept up by the rear wing, even without monkey seats or Renault trick, the cooling is exhausted in a similar region so will be influenced in a similar way, maybe not quite to the same extent. That leaves and brakes/tyres - brakes peak at 1000°C, but will average to about the same as the exhaust around the lap, plus tyre surfaces are 80-100°C depending on the compound.

It will certainly have an effect on a following car, how much I dunno. A 10°C increase of temperature does not give a massive drop in air density, it's certainly smaller than the effect of altitude at e.g. Mexico vs sea level, plus it will dissipate further back from the car.

[henry]

1250 kW comes from fuel with energy density 45 mJ/kg at 100 kg/hr

Some claim a slightly higher energy density.

[DiogoBrand]

@jjn912

Do you think the heat rejected out of the back of the car has an effect?

The exhaust and cooling vents are outputting around 700 kW into the area bounded by the floor, wing and rear tyres, plus some outboard from the brakes. I did a cursory calc and got an average 10°C rise for a plume of car cross section at 60 m/s Since this will lower the air density I assume it reduces downforce.

Wouldn't 700kW of heat mean 0% engine efficiency?

No.

The fuel flow rate gives around 1250kW (Andy Cowell's number)

If we assume 50% efficiency that’s 625 not leaving the crank but exiting out of the exhaust and radiators. Of the 625 that does some is lost in the transmission, some in the CU, some in the ES etc.

I used 700 because it’s a nice round number. It might be 600 or 800 doesn’t make a huge difference.

Big math mistake on my part. I was considering that if you had 700kW of wasted energy you'd have none left as mechanical power, when in fact the engine power and wasted energy both add up into the total fuel energy.
Thank you for clarifying.

[henry]

...

It will certainly have an effect on a following car, how much I dunno. A 10°C increase of temperature does not give a massive drop in air density, it's certainly smaller than the effect of altitude at e.g. Mexico vs sea level, plus it will dissipate further back from the car.

The difference is that at altitude leading and following car experience the same loss.

The key issue is following closely, less than a second, 60m, and hopefully a couple of car lengths, 30m. The claim is that currently 1.5 to 2 seconds is as close as they can get.

Another thought. The road surface is usually quite a bit hotter than the ambient air. I assume their will be a boundary layer of air which is rapidly heated by the road. This air will be streamed by the front wing and underbody to exit at diffuser level.

[godlameroso]

Little known fact, the kinematic viscosity of air increases with temperature. It tends to "stick" more as it's hot, so even though you lose volume it's somewhat but not completely counteracted by that. Part of the reason exhaust blowing works so well is that the heat reduces the likelihood of flow separation over a surface, and if you can get it in the center of the vortex will actually add energy to it.

[jjn9128]

The difference is that at altitude leading and following car experience the same loss.

Sorry that wasn't my point, it was just that it's a small change and that seemed a good comparison, but yes obviously.

[3jawchuck]

Little known fact, the kinematic viscosity of air increases with temperature.
....

Isn't that because it depends on density?

[godlameroso]

Little known fact, the kinematic viscosity of air increases with temperature.
....

Isn't that because it depends on density?

What do you mean?

[3jawchuck]

Little known fact, the kinematic viscosity of air increases with temperature.
....

Isn't that because it depends on density?

What do you mean?

I remember that kinematic viscosity is equal to dynamic viscosity over density. So all things being equal it would increase with temperature. However, this morning I bothered to look into it and learned that dynamic viscosity increases with temperature due to something called Sutherland's law.